This invention relates to microphones and more particularly relates to apparatus for reducing the response of a microphone due to mechanical shock.
Microphones which utilize a pneumatic pumping chamber in order to reduce the output of the microphone in response to mechanical shock have been devised in the past. One such microphone is described in U.S. Pat. No. 3,240,883 (Seeler -- Mar. 15, 1966). Microphones of the type described in the Seeler patent have been marketed under the tradename "Unidyne III" by Shure Brothers Incorporated, Evanston, Ill., since the early 1960's.
Referring to the Unidyne III microphone shown in FIG. 1 of U.S. Pat. No. 3,240,883, normal output occurs when sound pressure waves impinge on the front of a diaphragm 21 and exert a force. The sound waves also impinge on the rear of the diaphragm finding access thereto through sound entry 40, screen 39, passage 30 and apertures 41. The rear of the diaphragm, the housing 9, pole piece 14 and related components form a diaphragm chamber located to the rear of the diaphragm. The time delay and the delay in the acoustic network comprising apertures 41 and resistance ring 22 cause the force on the front of diaphragm 21 to be in advance of the force on the back of diaphragm 21. The force differential between the front and back of diaphragm 21 creates a net force which moves diaphragm 21 and an attached voice coil 20 in a magnetic air gap 16. If voice coil 20 does not move relative to gap 16, no output is produced.
This effect is used in two ways in the Unidyne III:
First, the microphone is made less responsive to sources of sound behind it than in front of it. If sound pressure from a source behind the microphone occurs and the delay in the internal acoustic network is equal to the external time delay between the rear sound entry and the front sound entry, the forces on each side of the diaphragm will be exactly in step and equal. Since they are in opposite directions, these forces will cancel each other, thereby reducing the output of the microphone.
Second, the microphone is made less responsive to axial mechanical vibrations. If axial mechanical vibrations arising from floor vibrations or handling shocks were transmitted freely to pole pieces 15, 18 of cartridge 10, the diaphragm-voice coil assembly 21, 20 would vibrate relative to magnetic air gap 16 like a weight suspended from a spring at a first resonant frequency. This would produce an undesirable output, and the microphone would be "vibration sensitive."
The situation is improved in the Unidyne III by placing a compliant support mounting, such as shock mount ring 25, between cartridge 10 and outer case 9. The effective mass of cartridge 10 and the compliance of ring 25 is designed to form a mechanical resonance at a second resonant frequency lower than the microphone frequency response range of interest. In addition, the second resonant frequency of the cartridge-outer case combination 9, 10 is placed lower in frequency (or pitch) than the first resonant frequency of the diaphragm-coil assembly 21, 20 so that shock-induced output at the first resonant frequency is reduced. However, the output at the second resonant frequency would continue to be troublesome if not corrected.
In the Unidyne III structure, the output at the second resonant frequency is reduced by the pressure changes generated in cavities 44 and 45. Shock mount ring 26 causes the volume of cavities 44 and 45 to change only in response to the component of accelerations impressed on case 9 normal to diaphragm 21. The volume of cavities 44 and 45 changes in the same direction as the diaphragm chamber and produces corresponding changes of pressure in cavities 44 and 45. These pressure changes are communicated to the rear surface of diaphragm 21 through an acoustic network including holes 46, the apertures in a pressure plate 33, a cloth washer 32, apertures 31, felt pad 30, cloth screen 28, passages 29, and air gap 16. The force produced by this pressure reduces the motion of voice coil 20 relative to magnetic air gap 16 and reduces the output due to axial vibration or shock.
In greater detail, the action is as follows:
Assume a shock force is applied to the left to outer casing 9 as shown in FIG. 1. In response to the shock force, outer casing 9 moves to the left. Some of the shock force is transmitted through resilient mountings 25, 26 to the cartridge 10 which, in turn, moves to the left. In addition, a further small fraction of the force is communicated to voice coil 20 through the flexible edge of diaphragm 21. Meanwhile, the inertia of cartridge 10 causes it to lag behind the movement of housing 9. This relative motion causes the cavities 44, 45 to be reduced in volume and, consequently, to slightly compress the air in the cavities. The increased pressure due to the air compression in cavities 44, 45 is communicated through the above-described acoustic network, and a fraction of the pressure is applied to the back of diaphragm 21. When the shock force is applied to the voice coil 20 through the flexible edge of diaphragm 21, the inertia of the voice coil causes its motion to tend to lag behind the motion of cartridge 10 and to create output due to relative motion. However, the force due to the pressure on the back of diaphragm 21 moves the diaphragm and voice coil to the left in step with the motion of cartridge 10 and magnetic air gap 16. As a result, a reduced output may be produced from voice coil 20 and magnetic air gap 16 in response to an axial mechanical shock.
A later shock-compensating microphone identical in principle to the above-described Unidyne III microphone is illustrated in U.S. Pat. No. 3,766,333 (Watson -- Oct. 16, 1973). Microphones of the type described in the Seeler and Watson patents are capable of shock cancellation in theory, but experience has shown that the principles described in these patents are difficult to apply in practice. The principal defect in these designs is their inability to provide uniform shock cancellation from one microphone to the next when the microphones are manufactured on an assembly line basis. It has been discovered that microphones capable of providing uniform shock cancellation can be produced by designing and balancing certain components inside the microphone according to the techniques described herein.
One feature of the invention contemplates the use of a microphone including an acoustical diaphragm having a first side and a second side with an effective area AD. The diaphragm vibrates in response to sound waves striking its surface. A voice coil having a mass MC is suspended on the diaphragm. A transducer having a mass MT converts the vibration of the diaphragm and voice coil into corresponding electrical signals. The transducer includes a housing for supporting the diaphragm in relationship to the transducer. An acoustical network having a complex acoustical impedance Z2 is coupled to the diaphragm. The acoustical network includes a variable volume cavity, a first channel having an acoustic resistance RB for couplng the variable volume cavity to the rear of the diaphragm and a second channel having a resistance RS for coupling the rear of the diaphragm to the atmosphere. The housing is supported within and resiliently coupled to an outer casing by a mounting assembly having a complex mechanical impedance of Z1 and an effective area AM.
If a shock force is applied to the outer casing in a direction parallel to the longitudinal axis of the microphone, the mounting assembly creates a pressure in the variable volume cavity which urges the diaphragm in a direction which tends to oppose the movement of the diaphragm due to the application of the shock force. As a result, the output of microphone due to the shock force is minimized.
It has been discovered that the shock sensitivity of a microphone of the foregoing type can be drastically reduced over a broad predetermined range of frequencies if the microphone components are designed to as nearly as possible achieve the balance condition defined by the equation: ##EQU1## and if impedance Z1 is adjusted relative to impedance Z2 so that the ratio ##EQU2## remains as nearly constant as possible over the predetermined frequency range and so that the phase angle of impedance Z1 and the phase angle of impedance (AM).sup.2 (Z2) are as nearly equal as possible.
According to another feature of the invention, the mounting assembly comprises an adjustable element which, in a preferred form includes a mounting diaphragm having a first side and a second side extending between the outer casing and the housing and an enclosed chamber defined in part by the second side of the mounting diaphragm. The enclosed chamber can be fitted with an adjustable air leak from the chamber to the atmosphere. By adjusting the volume of the chamber and the resistance of the air leak, the complex impedance of Z1 may be made to more nearly satisfy the above equation.
By using the foregoing techniques, it is possible to manufacture microphones with a degree of broad band shock cancellation uniformity previously unattainable.